Venturi Adjustment on Scuba Regulators (Dive/Pre-Dive, Off-On, and +/- Switch)

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The Pre-Dive/Dive, On/Off, or +/- Adjustment on a Scuba Regulator

zeagle second stage regulator for scuba diving
The red arrow shows the "Venturi switch" on my alternate air source. These adjustments may be found on the side or top of the regulator second stage. Natalie L Gibb

What do you notice about the design of a regulator second stage? At first glance a diver may notice the size, weight, or color. Perhaps you notice an interesting little knob on the second stage labeled "Dive/Pre-Dive," "On/Off," or "+/-". This switch or knob alters the airflow inside the regulator, making breathing either easier or more difficult. Turning the knob enables and disables something called the Venturi Effect, which regulator designers take advantage of to assist breathing. Click through the following pages to find out how it works, and when you should disable the Venturi Effect.

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What Is the Venturi Effect?

simple sketch of the venturi effect
Here's a silly sketch I made of the Venturi Effect. (Thank goodness I am writer, not an artist!) Airflow accelerates as air moves through a constriction. As it flows out of the constriction, it drags along other air particles, creating a low pressure area. Natalie L Gibb

The key to understanding how airflow can lower the work of breathing is a concept called the Venturi Effect. The Venturi Effect explains how fast moving air molecules can be used to create a vacuum. Here's how it works.

The Venturi Effect states that when air is forced through a constriction, such as the tiny valves inside a regulator second stage, the speed at which the air particles travel will increase.

When air exits the constriction, it is moving very rapidly in comparison to the surrounding air particles. The fast moving air draws some of the surrounding slow-moving air particles along with it.

Slow-moving air particles are continuously dragged away. This results in the lowering of air pressure (a vacuum) in the region surrounding the fast-moving airflow.

Some scuba regulators use the vacuum created by the Venturi Effect to reduce the work of breathing in scuba regulators. To understand this, let's first review the basics of second stage operation.

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Regulator Second Stage Function (Really) Simplified

drawing of a regulator second stage function
1. A simplified second stage diagram. 2. When a diver inhales, he applies suction on a flexible diaphragm which bends towards him (green arrow). The diaphragm presses a lever (green arrow), and the lever opens a valve allowing air to flow (blue arrows). Natalie L Gibb

A regulator second stage is a relatively simple machine. When a diver breathes in, his inhalation draws a flexible diaphragm inside the second stage towards him. As it moves, the diaphragm presses against a lever. This lever opens a valve to allow air to enter the second stage. When a diver stops inhaling, the diaphragm relaxes into its original position, releasing the lever and stopping the airflow.

In the most simple second stage designs, the diver must continue to inhale (relatively) forcefully against the diaphragm to keep the valve open and obtain a full breath. In reality, this inhalation is not difficult, and such simple regulators work perfectly well for most recreational diving applications. However, clever regulator designers figured out a way to make breathing even easier using the Venturi Effect.

More about scuba regulators:
DIN vs Yoke Regulators
What Is a Balanced Regulator?
Definition and Basic Parts of a Regulator

**Yes, I know the drawing is missing exhaust valves and other important parts. This is just to illustrate a concept as simply as possible. Plus, I am really not artistic, and exhaust valves, purge buttons, and realistic regulators are very hard to draw.

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Venturi-Assited Breathing

Drawing of a regulator with and without a venturi-assist.
Left: Airflows without a Venturi-assist device. Air squiggles out everywhere (blue). Right: A Venturi-assist can channel air in combination with molded contours inside the second stage, creating a low-pressure area (green). Natalie L Gibb

Some regulators are designed to take advantage of the Venturi Effect. Fast-moving air flowing into the second stage is channeled by a Venturi-assist device and modeled plastic contours in the regulator body. When directed properly, the fast moving air creates a vacuum behind the regulator's diaphragm due to the Venturi Effect (bright green star).

Here's how it works. A diver inhales normally, and the diaphragm flexes towards him, initiating airflow. Once the diver inhales and airflow begins, the same air that he is breathing creates a vacuum which helps to maintain the regulator diaphragm flexed towards the diver.

The force required to hold the diaphragm towards the diver and keep the valve open is partly supplied by the diver's inhalation, and partly by the Venturi Effect of the fast-flowing air.

Regulators with Venturi-enhanced performance require only the slightest inhalation to begin air flow, and are a pleasure to breathe from.

**Yes, I know the drawing is missing exhaust valves and other important parts. This is just to illustrate a concept as simply as possible. Plus, I am really not artistic, and exhaust valves, purge buttons, and realistic regulators are very hard to draw.

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The Downside of the Venturi Effect - Easy Free flow When Enabled

a scuba diver floats on a tropical ocean.
A diver who turns the Venturi adjustment on her regulator to "Pre-dive" or "Off" before removing her regulator from her mouth is unlikely to have a regulator free flow on the surface. ©

The main drawback of regulators that use the Venturi Effect to enhance breathing is that they have a tendency to free flow more easily than other regulators. Free flows caused by the Venturi Effect may occur anytime the second stage is out of the diver's mouth and airflow is triggered.

One example is the common situation in which a second stage is dropped into the water mouthpiece-up. The water pressure on the purge button initiates airflow. Once air begins to flow into the second stage, the vacuum created by the Venturi Effect sucks the diaphragm towards the mouthpiece, and air flow will continue until the diver acts to stop it.

A free flow related to the Venturi Effect is not a cause for alarm. It does not indicate a problem with your regulator. However, the free flow must be stopped to avoid significant air loss from the tank. A diver can easily stop the free flow by turning the regulator mouthpiece-down in the water or by placing a finger across the opening of the mouthpiece (among other methods). Any method that alters the air flow or allows pressure to build up inside the second stage will stop a Venturi-related free flow.

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How to Avoid Free Flow Caused by the Venturi Effect

Photo of the Dive/Pre-Dive Adjustment on a Mares regulator
The Mares Prestige-22-DPD regulator's Venturi adjustment. On this regulator, the diver turns the knob to "Dive" to enable Venturi-assisted breathing, and turns it in the opposite direction to disable the effect while on the surface. © Mares 2012

Regulators that make use of the Venturi Effect to reduce breathing resistance normally have a switch on the second stage body with two positions, a Venturi-enabled setting and a Venturi-disabled setting (which alters airflow within the second stage body). These "Venturi switches" are commonly labeled "dive/pre-dive" "on/off" and "+/-" depending upon the regulator brand and model.

To avoid free-flow caused by the Venturi Effect, deactivate Venturi-assisted breathing by moving the switch to the appropriate position (pre-dive/off/-) until you begin breathing from the regulator. Be sure to disable the Venturi Effect whenever the regulator is out of your mouth, and be certain to keep your alternate air source regulator's Venturi switch in the disabled position. Deactivating Venturi-assisted breathing does not alter the regulator's ability to give you air, but the regulator will breathe slightly "harder" until you re-enable the Venturi effect.

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The Take-Home Message About Venturi Adjustments on Regulators

a mother and son scuba diving team enter the water
Now you know how (and why) you should adjust your regulator on the surface. Turn your regulator to "Pre-Dive" whenever entering the water and you should avoid most Venturi-related free-flows. ©, Jman78

Many scuba regulators make use of the Venturi effect to reduce breathing resistance. Such regulators are a delight to breathe from. Just be sure to turn the Venturi switches on both your primary and your alternate air sources to the "Pre-Dive" setting whenever the regulator is out of your mouth.

Regulator-related dive skills:
Regulatory Recovery - Find a Lost Reg
• Free Flow Regulator Breathing
• Should You Remove Your Regulator From Your Mouth During an Emergency Ascent?